kconfig: fix savedefconfig to handle TPL correctly

Since 3ff291f3
(kconfig: convert Kconfig helper script into a shell script),
"make savedefconfig" of TPL boards has not been working.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.o.rg>
Acked-by: Simon Glass <sjg@chromium.org>